Optoelectronic devices are used to convert between electrical and optical signals. For example, in fiber optic communications, optical modulators are used to convert an electrical information carrying signal into an optical modulated signal. In an optical modulator, an electrical signal is applied to a material having an optical property dependent on an electric field or an electric current within the material. A light wave traveling through the material is thus modulated by the electrical signal. To improve the efficiency of modulation while keeping a high modulation frequency, the light wave and the electrical signal (a radio-frequency electromagnetic wave) can be made to co-propagate in the material. Optoelectronic devices employing co-propagation of light and electrical signals belong to a class of so called traveling-wave devices.
Referring to FIG. 1, a prior-art traveling-wave optical modulator 100 is shown. The traveling-wave optical modulator 100 includes two waveguides 102 and 104 formed in an electro-optic crystal such as lithium niobate, and two 3 dB couplers 106 and 108 forming a Mach-Zehnder interferometer 110, an interaction region 112 along the lower waveguide 104, the interaction region 112 defined by two electrodes 114 and 116, and a terminating resistor 118 coupled to the lower electrode 116. In operation, an optical signal (an optical wave) 120 is provided to an input port 122 of the optical modulator 100. A driver 124 generates an electrical modulating signal 126 applied to the lower electrode 116. The optical signal 120 and the electrical signal 126, in form of a radio-frequency (RF) electromagnetic wave, co-propagate in the interaction region 112. The RF wave 126 causes a slight modulation of the refractive index of the lower waveguide 104. A wave of the refractive index modulation travels with the speed of the RF wave 126 in the interaction region 112. The refractive index modulation results in a change of a phase of the co-propagating optical wave 120. The change of the phase of the optical wave 120 is translated into a change of optical power of the optical signal at the output 3 dB coupler 108. The intensity-modulated optical signal 120 exits the traveling-wave optical modulator 100 at an output port 128 thereof. The electrical modulating signal 126 is terminated by the terminating resistor 118 having a real impedance matched to that of a RF transmission line formed by the electrodes 114 and 116. The impedance is matched to prevent undesirable reflections of the modulating RF wave 126 back into the driver 124.
One important characteristic of the prior-art modulator 100 is a frequency response function (or so-called “S21” function). The frequency response function is a degree of modulation of the optical signal 120 as a function of frequency of the electrical signal 126. For the prior-art modulator 100 to produce a high-quality, low jitter optical modulated signal, the frequency response function has to be as smooth and even as practically achievable. Detrimentally, the frequency response function of the prior-art modulator 100 usually has a spectral ripple due to parasitic electrical couplings and acoustic resonance effects caused by electrostriction in the electro-optic crystal the waveguides 102 and 104 are formed in, or more specifically, in the interaction region 112 of the crystal. This spectral ripple is difficult to remove, because the electrostriction in electro-optic crystals has the same physical origins as the electro-optical effect used to effect the phase modulation on the optical signal 120.
The problems of spectral ripple and a roll-off of the frequency response function of an optical modulator are well recognized in the art. A number of approaches aiming to reduce the spectral ripple and flatten the frequency response function have been suggested.
One approach is to provide a custom front-end electrical filter 130 to compensate for undesired spectral features in the frequency response function of the traveling-wave optical modulator 100, or to design a gain spectral characteristic of the driver 124 to mirror the undesired spectral features, so they can be subtracted. The latter approach is disclosed by Shimizu et al. in U.S. Pat. No. 7,558,444, incorporated herein by reference. Detrimentally, incorporating front-end filters, such as the filter 130 in FIG. 1, results in a reduction of the efficiency of modulation.
Nakajima et al. in U.S. Pat. No. 7,345,803, incorporated herein by reference, discloses a method of correcting a high-frequency roll-off of a response function of an optical modulator by providing an inductance connected in series or in parallel to the RF transmission line of the optical modulator. The inductance effectively alters the impedance of a termination circuit, which can reduce the roll-off of the response function. Detrimentally, the technique of Nakajima does not address a problem of acoustically caused ripple in the response function, because of the narrowness of the spectral features caused by acoustic resonances in the electro-optic crystal.
Other approaches to reduce acoustically caused ripple and improve overall flatness of the response characteristic include lowering the resistance of the terminating resistor 118; doping the electro-optic crystal; providing a resistive conformal coating on the electro-optic crystal; or altering geometry of the electrodes 114 and 116. For example, Skeie in U.S. Pat. Nos. 5,854,862; 5,675,673; 5,671,302 incorporated herein by reference; and Dolfi et al. in U.S. Pat. No. 5,138,480, incorporated herein by reference, disclose traveling wave optical modulators, which have segmented electrodes of a complex spatially varying shape. Detrimentally, these approaches result in raising a magnitude of the electrical signal 126 required to drive the traveling-wave optical modulator 100.
The prior art is lacking a technique that would allow one to inexpensively and effectively reduce or suppress detrimental spectral ripple of the response function of an optoelectronic device. Accordingly, it is a goal of the present invention to provide such a technique and a device.